The present invention relates to a magnetic detection apparatus for use in a rotary encoder, a linear encoder, etc.
There has been well known a rotary encoder for detecting a rotation angle of a rotation axis, in which the rotation angle is detected by reading magnetic signals previously recorded in the form of a magnetization pattern having a constant bit length on a magnetic record medium applied on a circular plate or a cylinder coupled with the rotation axis by means of a magnetic sensor including a magnetoresistive (MR) element. However, when the rotation angle of a motor is detected by the conventional rotary encoder, there occurs drawbacks that the S/N ratio of a detection signal supplied from the magnetic sensor is decreased and an erroneous operation is effected due to an external magnetic field, because a large amount of the external magnetic field leaks from a permanent magnet or an armature of the motor.
In order to eliminate the drawbacks mentioned above, there has been proposed, in Japanese Patent Laid-Open Publication No. 162,556/79, an angle detector comprising a magnetic shielding member made of high permeable magnetic material for surrounding a circular plate including a magnetic record medium and coupled with an output axis of the motor, a magnetic sensor arranged opposite to the circular plate and driving circuit and signal processing circuit connected to the sensor. However, it is not possible to shield the angle detector completely from the external magnetic field. Moreover, in order to remove the influence of the external magnetic field, it may be further considered that only the external magnetic field is detected by a separate magnetic sensor and the thus detected signal and the output signal derived from the magnetic recording medium are electrically calculated. However, in this case, since a magnetic-resistance characteristic of the MR element consisting of the magnetic sensor is not linear, the output signal of the separate magnetic sensor due to the external magnetic field does not correspond to a part of the output signal of the other sensor due to the external magnetic field, and thus it is not possible to perform a highly accurate compensation. Moreover, the dynamic range of the MR element could not be used effectively for detecting the magnetic pattern, particularly under the large external magnetic field.
In the rotary encoder mentioned above, it is possible to detect the magnetic signals by a single MR element so as to obtain the displacement amount. However, since the output voltage is small and drifts due to the temperature variation, it is general to derive the output voltage from a circuit wherein more than two MR elements are differentially connected. For example, in Japanese Patent Laid-Open Publication No. 115,257/79, is disclosed an angle detector wherein two MR elements are arranged by a distance which is an integral number of a pitch of the magnetization pattern on the magnetic recording medium and an output difference therebetween is derived from a differential amplifier. Moreover, an angle detector of this type shown in FIG. 1 is described in a Japanese magazine, "Nikkei Electronics", page 88 published on June 22, 1981. As shown in FIG. 1, two groups of magnetic sensors each having four MR elements A.sub.1 to A.sub.4 and B.sub.1 to B.sub.4 are arranged. Then, MR elements in one group are arranged apart by a half of a pitch P of a magnetization pattern recorded on a magnetic recording medium M, and MR elements in the other group are arranged apart by P/4 with respect to the MR elements in the other group. Further, as shown in FIG. 2, the four MR elements of each groups are connected as a bridge circuit, and a difference between output voltages generated at diagonal points of each bridge circuit are derived from respective differential amplifiers DA.sub.1 and DA.sub.2. In this magnetic detector, it is possible to detect not only a displacement amount, but also a displacement direction. Moreover, it is possible to make an output amplitude large and to remove the effect of the drift. However, in the conventional angle detector, it is necessary to arrange more than two MR elements apart from each other in a displacement direction D, i.e. in an arranging direction of the magnetization pattern by an interval which is equal to an integer multiple of the pitch P of the magnetization pattern or to a reciprocal thereof. Therefore, when use is made of various magnetization patterns having different pitches, it is necessary to prepare various magnetic sensors having MR elements which are arranged apart by predetermined intervals corresponding to the various pitches of the magnetization patterns. Therefore, there occurs a drawback that the freedom of design is limited to a great extent. Moreover, in case of arranging the magnetic recording medium on the cylinder side surface, if a plurality of MR elements are arranged on a plane substrate, the distances between each MR element and the magnetic recording medium are not equal. Therefore, the output signal of each MR element is fluctuated due to the distance variation, and thus the differential output includes an error.
In order to eliminate the drawbacks mentioned above, there has been proposed in Japanese Patent Laid-Open Publication No. 35,011/81 an angle detector wherein a width of each MR element is varied, but it is a very cumbersome task to manufacture the MR elements each having a different width. Moreover, if the distance between the MR element and the magnetic recording medium is varied as mentioned above, it is necessary to arrange the MR elements each having a different width corresponding to such distance. In addition, if more than two MR elements are arranged in the direction of the magnetization pattern recorded on the magnetic recording medium, a dimension of the magnetic sensor is made large and thus the detection apparatus is liable to be large in size correspondingly.